The present invention relates to the production of secondary monoalkyl sulfuric acids and their salts. Such substances are widely known and have a broad range of recognized utility. For instance, the acids have been employed both as intermediates and as catalysts in organic synthesis and further as solvents in various formulations. The salts, particularly the sodium salts of secondary C.sub.8 to C.sub.22 monoalkyl sulfuric acids, are well known compounds with established utility as detergent components.
It is known to prepare monoalkyl sulphuric acids by reacting one or more C.sub.8 to C.sub.22 olefins, which may be internal or alpha-olefins, with sulfuric acid, said acid having a concentration of 75 to 100 percent by weight (%w). To prepare the corresponding salts the acids thus produced are neutralized with suitable bases, such as amines, or such as ammonium, alkali metal, or alkaline earth metal hydroxides, carbonates, or bicarbonates.
These two main reactions, i.e. sulphation and neutralization, may be illustrated by the following equations representing the preparation of secondary sulphate sodium salts from C.sub.8 to C.sub.22 alpha-olefins: ##STR1## wherein R is an alkyl group of from 6 to 20 carbon atoms.
It is further known (U.S. Pat. No. 2,623,894; U.S. Pat. No. 2,640,070; British Pat. No. 691,929) that certain oxygen-containing compounds, present in the feed to the sulphation reaction in amounts up to 10%w, based on the weight of olefins in this feed, have a beneficial effect on the reaction rate or yield of monoalkyl sulphuric acids. Such oxygen-containing compounds include aliphatic alcohols, aldehydes, ketones, ethers, ether alcohols, carboxylic acids, sulphurous esters, and alkylphenol/alkylene oxide adducts. However, the production of monoalkyl sulphuric acids and their salts through these conventional processes is characterized by the formation of large quantities of secondary dialkyl sulphates (DAS) in the sulphation reaction. Although DAS formation may be suppressed by the presence in the sulphation reaction of the oxygen-containing compounds listed above, the quantity of DAS formed is still by no means fully acceptable. For example, when the sulphation reaction is carried out in the presence of aliphatic alcohols in amounts of up to 10%w, based on the weight of olefin, DAS is formed in amounts usually exceeding 40 mole percent (% mole), based on the monoalkyl sulphuric acid produced. DAS may be formed according to the following equations: ##STR2##
When it is desired to produce the monoalkyl sulfuric acid salts (MAS) from the acids, this problem of DAS formation is partly overcome by hydrolysis or saponification of the DAS to the desired MAS, suitably during the subsequent neutralization reaction, according to the equation: ##STR3## However, it can be seen that this is still only a partial solution to the problem since the hydrolysis also produces one mole of secondary alcohol which represents a loss of olefin. A further disadvantage of this known process for salt manufacture is that an amount of secondary alcohol is formed which is in excess of that which can be explained as a result of DAS hydrolysis. It is considered that this excess amount results from the hydration of olefins to alcohols during the sulphation reaction. The alcohols may, according to British Pat. No. 691,929, be recovered following neutralization of the monoalkyl sulfuric acids and used in the sulphation reaction, but while the amount thereof in the feed to the sulphation reaction is maintained below 10%w, on olefin, there will always ultimately be an excess of alcohol which has to be disposed of. One method of disposal (British Pat. No. 656,064) involves dehydration of the recovered alcohols to form olefins and the subsequent use of these olefins in further sulphation reactions. However, this method necessitates a separate dehydration reactor.
Thus, it would be highly desirable to reduce or eliminate by-product DAS and alcohol formation during the production of monoalkyl sulphuric acids and their salts.